There has come into general use a class of synthetic polyurethane which is formed by the reaction of two highly reactive chemicals, typically an organic resin and a polyisocyanate. These chemicals are relatively stable when alone, but when mixed in the proper proportion, react within a very few seconds to form a polyurethane foam.
Polyurethane foam can be used in a variety of applications, including insulation and foam-in-place cushioned packaging material. The fast reaction and setting of the chemicals to form the polyurethane foam is highly desirable from the standpoint of high production rates of the foam, but reaction of the chemicals inside the dispensing equipment can clog and jam the internal parts of the dispensing equipment, rendering the equipment inoperative.
To alleviate some of the problems of mixing and dispensing of polyurethane foam precursor chemicals, several different approaches to the design of dispensing equipment for polyurethane foams have been suggested.
A type of polyurethane foam dispensing equipment in wide use is exemplified in U.S. Pat. No. 3,263,928. The device disclosed in this patent includes a mixing chamber assembly having a polytetrafluoroethylene (available as "Teflon" from DuPont Corporation) core circumferentially and axially contained within a metallic housing. The Teflon core has an axial bore therethrough with individual inlets therein for the injection of the reactive chemicals from respective sources of supply into the bore and a valving rod reciprocally movable within the bore. The Teflon core is maintained under a compressive force by a threaded nut disposed at the rear of the mixing chamber core to keep an interference fit between the valving rod and the Teflon core to maintain proper sealing of the valving rod with the inlets.
The valving rod is connected to an air piston which is actuated by means of a trigger. When the trigger is depressed, the valving rod is reciprocated back to uncover the inlets, thereby allowing entry of the reactive chemicals into the bore for mixing and dispensing out the front of the bore. After the desired quantity of the mixture has been dispensed, the rod is reciprocated forward to close off the ports to prevent further entry of the reactive chemicals into the bore. The valving rod is then moved still forward to the end of the bore so as purge any mixed chemicals which remain in the mixing chamber bore.
This design has met with a limited degree of success, but has many drawbacks. Despite the interference fit between the valving rod and the bore, as the valving rod is actuated a number of times, a gradual build-up of reacted polyurethane material on the valving rod occurs. Even though Teflon is an inherently low friction material, as polyurethane is built-up on the valving rod, the friction between the valving rod and the core may increase to the point where the valving rod becomes jammed in the bore and cannot be reciprocated by the air cylinder. When this happens, the dispensing equipment is rendered inoperative and must be disassembled and cleaned to free the valving rod. In time, the Teflon core itself may become unserviceable due to wear. In this case, after removal of the valving rod from the Teflon core, the Teflon core itself must be removed from the housing and replaced, which is a laborious process. During such repair, the apparatus is unusable.
Other proposals for minimizing jamming of the valving rod in the bore are disclosed in U.S. Pat. Nos. 3,687,370 and 4,023,733. These dispensing devices have a Teflon mixing chamber core with a bore therethrough fitted into a metallic housing. The disclosed devices also includes a valving rod tightly fitted into the bore and reciprocal to selectively seal or open the ports, similar to the operation shown in U.S. Pat. Nos. 3,263,928, except without means for axially compressing the Teflon core.
The devices disclosed in U.S. Pat. Nos. 3,687,370 and 4,023,733 propose to minimize jamming by means of a reservoir containing a solvent material positioned behind the mixing chamber. When the rod is reciprocated to the rearward position, the back of the rod is bathed in the solvent, which acts to prevent the reaction of the precursor chemicals and dissolve built-up polyurethane, which is said to reduce the tendency of the valving rod to jam in the Teflon mixing chamber. However, jamming can still occur, and such jams necessitate disassembly and cleaning of the equipment. In some cases, the jamming may be so serious as to require removal and replacement of the Teflon core from the housing, which is a time consuming and laborious process requiring special equipment. During such repair or replacement, the apparatus is unusable. Furthermore, the solvent itself is toxic.
U.S. Pat. Nos. 4,469,251 and 4,568,003 disclose a dispensing device having a detachable mixing chamber assembly. The mixing chamber assembly has a Teflon core having a bore and a valving rod in an interference fit in the bore, similar in function and operation to the device disclosed in U.S. Pat. No. 3,263,928. The Teflon core is axially and circumferentially retained in a nondeformable housing, which is also part of the detachable mixing chamber assembly. This housing axially and radially contains and compresses the Teflon core to maintain the interference fit.
Belleville spring washers are also provided at one end of the housing to maintain the Teflon under a compressive force. This, it is said, maintains the interference fit between the valving rod and the bore so as to maintain an effective seal between the valve and the bore to prevent leakage. These patents further teach use of nondeformable inserts in the inlets to maintain the shape of the inlets during use. However, jamming may still occur, and in this event, the mixing chamber, and hence the device, becomes unusable.
The devices disclosed in U.S. Pat. Nos. 4,469,251 and 4,568,003 have two separable portions. One portion includes a mixing chamber assembly, including the nondeformable housing, Teflon core, nondeformable inserts, valving rod and Belleville washers; and another portion includes means for reciprocating the valving rod. When the mixing chamber assembly becomes jammed or otherwise fails to function, the entire mixing chamber assembly, including the nondeformable housing, Teflon core, nondeformable inserts, valving rod and Belleville washers, is removed from the other portion of the apparatus and replaced with a fresh assembly.
While ostensibly an improvement over the type of dispenser disclosed in U.S. Pat. Nos. 3,263,928; 3,687,370 and 4,023,733, the apparatus disclosed in these patents has serious disadvantages. Use of a reciprocal rod in the bore of the Teflon core as the valving means for the inlets is undesirable. Because the chemicals are maintained under pressure directly adjacent the valving rod, unless the core is kept under a high compressive force to maintain a tight interference fit between the valving rod and the bore, the chemicals have a tendency to leak around the valving rod when the valving rod is in the forward position, causing premature reaction of the chemicals and thus contributing to jamming and clogging of the inlets. Furthermore, the relatively high compression necessary to maintain sealing of the valving rod in the Teflon core increases the friction between the valving rod and the Teflon core, and thus the force required to reciprocate the valving rod in the bore. Still further, the compression on the core has a tendency to cause the nondeformable inserts in the inlets to back away from the rod to the extent permitted by the nondeformable housing. This can potentially allow the Teflon material of the core itself to extrude underneath the discharge end of the insert to occlude the inlets.
Teflon is also a material which exhibits a tendency to "cold-flow". Thus, Tefon when placed under a compressive force will continue to deform over time. The tight compressive force necessary between the mixing chamber core and the associated valving rod has a tendency to compress the Teflon radially inwardly. In time and after a number of valve rod reciprocations, the inner surface of the core is shaved somewhat, destroying the integrity of the tight interference fit.
Furthermore, the mixing chamber assembly with its non-deformable housing, Belleville washer assembly for maintaining the Teflon core under compression, Teflon core, nondeformable inserts and valving rod is relatively expensive. Discarding the entire mixing chamber assembly is undesirable because of the expense, particularly since the failure is usually confined to only the Teflon core itself and valving rod (which are themselves inexpensive). However, because the Teflon core is tightly contained within the circumferential housing, removal and replacement of the Teflon core and valving rod alone from the housing is difficult and requires special equipment. Still further, the mixing chamber has a limited life, and there is no means of determining when the mixing chamber has nearly reached the end of its useful life. The only way to determine when to replace the mixing chamber assembly is to wait until a failure of some kind, such as jamming or failure to produce acceptable foam, occurs. This is undesirable since failures of any kind occur as unpredictable times and can have costly results.
Another proposed design of reactive chemical device is disclosed in U.S. Pat. No. 4,133,483. This patent discloses a dispensing device having a mixing chamber and inlets for the chemicals leading into the mixing chamber. Separate valves are provided for each inlet, said to be operable substantially simultaneously by a manual hand lever. A purging rod which is operable completely separate from and independent of the valving means for the chemicals is provided to purge out reactive chemical after dispensing is complete, but which is not utilized to open or close the inlets. Because the purging rod is independent of the valving means, it can be operated several times, if necessary, to purge out remaining chemicals without activating the mixing and dispensing operation.
The device disclosed in U.S. Pat. No. 4,133,483 has several disadvantages. It has been found that it is very difficult in practice to effect simultaneous and complete opening of the separate valves with the manual lever because the human hand is too slow. And if the operator only partially depresses the lever, the valve for one chemical may open completely while the other valve opens only partially, resulting in an improper ratio of the mixture. Furthermore, because the purging operation is separate and independent from the valving dispensing operation, a careless operator can forget to purge after dispensing, resulting in a jammed and clogged dispenser.